Loop heat pipe and electronic apparatus

ABSTRACT

An evaporator of a loop heat pipe includes a case having a liquid flow inlet and a vapor flow outlet, and at least one porous body disposed inside the case and configured to guide liquid-phase working fluid inward of the case. The evaporator further includes a liquid supply duct disposed inside the case and configured to guide the working fluid into the porous body from the liquid flow inlet. The liquid supply duct is made of a material having lower heat conductivity than a material of the case. The working fluid having flowed into the evaporator is prevented from evaporating before reaching the porous body.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of PCT International Application No. PCT/JP2010/068041, filed on Oct. 14, 2010, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a loop heat pipe and an electronic apparatus.

BACKGROUND

Loop heat pipes (LHP) are known as devices for cooling various types of heat generating elements. In a loop heat pipe, an evaporator and a condenser are connected in a loop using a vapor transport line and a liquid transport line. Liquid-phase working fluid evaporates in the evaporator due to heat supplied from a heat generating element, and the vaporized working fluid is transported to the condenser in which the vapor condenses back to liquid by heat dissipation. The evaporator vaporizes the working fluid to transfer heat from the heat generating element (the heat of vaporization), and also functions as a pump for driving the circulation of the working fluid.

FIGS. 1A through 1C illustrate a conventional evaporator 10. FIG. 1A is a schematic cross-sectional view of the evaporator 10 along the direction of working fluid flowing from a liquid transport line to a vapor transport line. FIGS. 1B and 1C are schematic cross-sectional views taken along the A-A′ line of FIG. 1A, and illustrate two conventional evaporator structures, a cylindrical type and a plate type, respectively.

The evaporator 10 includes a metal case 20 connected to a liquid transport line 50 and a vapor transport line 55; and a wick 30 which is a porous body disposed inside the metal case 20. Working fluid 60 a supplied through the liquid transport line 50 flows into a liquid supply path 31 disposed substantially in the center of the wick 30 and is guided to the inner wall of the metal case 20 by capillary force in pores of the wick 30, which is a driving force for the working fluid 60 a. Subsequently, the working fluid 60 a is vaporized by heat transferred to the metal case 20 from the heat generating element, to thereby form vapor 60 b. The vapor 60 b then passes through vapor discharge grooves 32 provided on the outer periphery of the wick 30, or on the inner wall of the metal case 20, and flows into the vapor transport line 55.

Application of loop heat pipes to cooling of electronic components, for example, central processing units (CPU) of computer systems, has been studied in recent years. Many electronic components have a flat heat dissipation surface, as represented by large scale integrated (LSI) packages. In the case of a cylindrical evaporator 10′ as illustrated in FIG. 1B, a flat plate 28 serving as a heat receiving surface is mounted on the case 20 of the evaporator 10′ in order to enhance contact of the case 20 with the heat dissipation surface. On the other hand, in the case of a plate-type evaporator 10″ as illustrated in FIG. 1C, one surface 29 of the case 20 generally having a rectangular parallelepiped shape is used as a heat receiving surface.

In general, in order to improve cooling performance of the loop heat pipe, increasing the internal volume of the evaporator is effective. On the other hand, the evaporator needs to be as compact as possible for producing a smaller and lighter electronic apparatus. In order to increase the internal volume while maintaining a compact, especially thin configuration, a plate-type evaporator, as illustrated in FIG. 1C, is considered to be preferable. To improve the cooling performance, using the evaporator case made of a metal, especially a metal with high heat conductivity such as copper, is effective. This is because the metal evaporator case facilitates heat transfer from the heat generating element to the entire outer periphery of the wick, which in turn facilitates vaporization of the working fluid. The metal evaporator case is preferable also in terms of providing sealing reliability to prevent the working fluid contained in the sealed evaporator case from leaking out.

However, downsizing of the evaporator may cause a problem as illustrated in FIG. 2. Heat is transferred from a heat generating element 70 to a part of the evaporator case 20, adjacent to the liquid transport line 50 (hereinafter, simply referred to as the “adjacent part”). This may heat the working fluid 60 a before the working fluid 60 a reaches the wick 30 after flowing out of the liquid transport line 50. Due to the heat, the working fluid 60 a may come to a boil at the adjacent part, which produces air bubbles 60 c. As illustrated in an enlarged schematic diagram of FIG. 2, the air bubbles 60 c enter the liquid supply path 31 of the wick 30, which leads to a condition where a gas phase exists on both sides of the wick 30. This creates a surface tension force 36 that cancels out a usual surface tension force 35 toward the outer periphery of the wick 30. The cancelling out of the surface tension forces 35 and 36 results in a loss of capillary force of the wick 30. In addition, the occurrence of the air bubbles 60 c raises the internal pressure of the liquid supply path 31, which may block the inflow of the working fluid 60 a from the liquid transport line 50. As a result, the circulation of the working fluid 60 a is reduced or stopped, which in turn leads to reduced cooling performance and/or operational instability of the loop heat pipe. These problems are likely to occur not only in downsizing of the evaporator but also in the use of a plate-type evaporator having a case, a part of which is used as a heat receiving surface, and/or in the use of an evaporator case made of a metal with high heat conductivity.

In view of the above-described problems, a technology has been proposed in which an end face of a cylindrical evaporator case, at which a liquid transport line is connected to the evaporator case, is made of a metal with relatively low heat conductivity or a resin to thereby prevent heat of the evaporator case from being directly transferred to the liquid transport line. However, using a resin as a material of the evaporator case presents problems in terms of pressure resistance and long-term sealing reliability. In the case of using a metal with low heat conductivity, the heat conductivity of such a metal is still several tens to several hundreds times that of a resin. Accordingly, a low-heat-conductive metal evaporator case does not provide sufficient heat insulation and, therefore, has limitations to sufficiently control a reduction in cooling performance.

-   [Patent Document 1] Japanese Laid-open Patent Application     Publication No. 2004-218887 -   [Patent Document 2] Japanese Laid-open Patent Application     Publication No. 2009-115396 -   [Patent Document 3] Japanese Patent No. 3591339

Consequently, demand is still being raised for technology capable of preventing the working fluid from evaporating before reaching the wick and avoiding a reduction in cooling performance and/or operational instability of the loop heat pipe.

SUMMARY

According to one aspect, there is provided a loop heat pipe with an evaporator which includes a case provided with a liquid flow inlet and a vapor flow outlet; and at least one porous body disposed inside the case and configured to guide liquid-phase working fluid inward of the case. The evaporator further includes a liquid supply duct disposed inside the case and configured to guide the working fluid into the porous body from the liquid flow inlet. The liquid supply duct is made of a material having lower heat conductivity than a material of the case.

According to another aspect, there is provided an electronic apparatus including the above-described loop heat pipe; and an electronic component thermally bonded to the evaporator of the loop heat pipe.

The object and advantages of the disclosure will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic cross-sectional view of a conventional evaporator;

FIG. 1B is a schematic cross-sectional view of a conventional cylindrical evaporator;

FIG. 1C is a schematic cross-sectional view of a conventional plate-type evaporator;

FIG. 2 is a schematic cross-sectional view illustrating a problem of the conventional evaporator;

FIG. 3A is a perspective view of components of an evaporator included in a loop heat pipe according to an embodiment;

FIG. 3B is a perspective view of a manifold of FIG. 3A, viewed in a different direction;

FIG. 3C is a cross-sectional view of the evaporator including the components of FIG. 3A, along a flow direction of working fluid;

FIG. 3D is a cross-sectional view of the evaporator taken along a B-B′ line of FIG. 3C;

FIG. 4 is a perspective view of components of an evaporator included in a loop heat pipe according to another embodiment;

FIG. 5 is a cross-sectional view of the evaporator included in the loop heat pipe according to the other embodiment;

FIG. 6A is a perspective view illustrating an electronic apparatus according to an embodiment;

FIG. 6B is a cross-sectional view taken along a C-C′ line of FIG. 6A;

FIG. 6C is a cross-sectional view taken along a D-D′ line of FIG. 6A;

FIG. 7 provides schematic cross-sectional views illustrating structures of application examples; and

FIG. 8 is a graph of evaluation results of the structures of FIG. 7.

DESCRIPTION OF EMBODIMENTS

Several embodiments are described in detail below with reference to the accompanying drawings, wherein like reference numerals refer to like elements throughout. Note that, in the drawings, the relative size ratio of various components may not be accurate, and dimensions are not to be scaled from the drawings.

Referring to FIGS. 3A through 3D, the following describes an evaporator 110 included in a loop heat pipe according to one embodiment. FIG. 3A is an exploded view of main components of the evaporator 110, and FIG. 3B illustrates a manifold 140 of FIG. 3A, viewed in a different direction. FIG. 3C is a cross-sectional view of the evaporator 110 including the components of FIG. 3A, along a flow direction of working fluid, and FIG. 3D is a cross-sectional view taken along the B-B′ line of FIG. 3C. It should be noted that the cross-sectional view of FIG. 3C does not depict a section made by one plane cutting the evaporator 110.

In the example of FIGS. 3A through 3D, the evaporator 110 is a plate-type evaporator. The evaporator 110 includes a first case part 121 communicating with a vapor transport line 155 at a vapor discharge outlet 126; a second case part 122 communicating with a liquid transport line 150 at a liquid flow inlet 125; two porous bodies (wicks) 130; and the branch tube (manifold) 140. The first and the second case parts 121 and 122 are coupled to each other to form a single evaporator case 120 for housing the wicks 130 and the manifold 140. The flat dimensions of the evaporator case 120 (i.e., dimensions of a heat receiving surface) are determined based on the size of a heat generating element, which is a cooling target. The thickness of the evaporator case 120 may be limited according to the packaging density of an electronic apparatus in which the evaporator case 120 is used. For example, in the case of applying the evaporator case 120 to an electronic apparatus with high-density packaging, such as a server and a personal computer (PC), the evaporator case 120 may need to be as thin as about 10 mm or less.

Two holes 123 for housing the two wicks 130 are provided in the first case part 121, as seen from the second case part 122 side. The shape of each of the holes 123 is determined by the profile of the wick 130 to be inserted thereto, but is typically round or oval. Between the two holes 123, a separation wall 124 is provided, connecting the top and bottom faces of the first case part 121. The second case part 122 may house the manifold 140. However, in this embodiment, no particular limitation is imposed on the structures of the two case parts 121 and 122 to form the evaporator case 120 as long as the case parts 121 and 122 can be coupled to each other after the wicks 130 and the manifold 140 are housed therein. For example, one of the case parts 121 and 122 may house both the wicks 130 and the manifold 140, and the other case part 121 or 122 may have a plate-like shape to form an end face of the evaporator case 120. Alternatively, the first and the second case parts 121 and 122 may have structures formed by splitting the evaporator case 120 into two in the thickness direction of the plate-type evaporator 110.

Each of the wicks 130 roughly has a cup-like shape, and has an opening defined by the inner periphery of the wick 130. The opening serves as a liquid supply path 131 for supplying the working fluid to the wick 130. The wicks 130 individually have multiple vapor discharge grooves 132 on the outer periphery. The grooves 132 may run the entire length of the wicks 130 along the flow direction of the working fluid. In that case, end portions of the grooves 132 on the liquid transport line 150 side may be terminated by the manifold 140, as illustrated in FIG. 3C. Note in FIG. 3C that the wick 130 and the evaporator case 120 are not in contact with each other because the cross-section of FIG. 3C is taken where the groove 132 is present. In a cross section of where no groove 132 is present, the wick 130 and the evaporator case 120 are in contact with each other. The wicks 130 are preferably resin wicks to be described later, and are formed to be individually larger than the internal dimensions of the corresponding holes 123 in the first case part 121 so as to be compressed when inserted into the holes 123. This enhances contact of the outer surfaces of the resin wicks 130 with the inner wall of the evaporator case 120, which may facilitate evaporation of the working fluid at the contact site of the wicks 130 and the evaporator case 120.

The average pore diameter of the wicks 130 is preferably 15 μm or less, and more preferably 5 μm or less, in order to obtain a sufficiently large capillary force. The wicks 130 preferably have high porosity so that the working fluid does not become insufficient at the contact site of the wicks 130 and the evaporator case 120. The porosity is set in the range of 30% or more and less than 90%.

The manifold 140 (to be described in detail later) insulates, from the evaporator case 120, working fluid 160 a having flowed in from the liquid transport line 150, and also serves as a liquid supply duct for supplying the working fluid 160 a to the individual wicks 130. The manifold 140 includes an inlet 143 provided corresponding to the liquid flow inlet 125 of the evaporator case 120; and two outlets 144 (see FIG. 3B) for supplying the working fluid 160 a to the two wicks 130. The manifold 140 has diverging flow paths 145 internally (FIG. 3D).

The manifold 140 is preferably formed to be in contact with the wicks 130 when housed in the evaporator case 120 so that the working fluid 160 a supplied from the liquid transport line 150 does not come in contact with the evaporator case 120 before reaching the wicks 130. However, covering a large part of the stretch from the liquid flow inlet 125 of the evaporator case 120 to the wick housing part of the evaporator case 120 by the manifold 140 may be enough to prevent the working fluid 160 a from coming to a boil before reaching the wicks 130. Accordingly, it may be acceptable to have a gap between the manifold 140 and the wicks 130.

The manifold 140 includes a main body 141 disposed inside the evaporator case 120 and may also include, on an as-needed basis, a tubular part (inner pipe) 142 extending inside the liquid transport line 150, as illustrated. The inner pipe 142 is preferably formed to be in close contact with the inner wall of the liquid transport line 150 so that the working fluid 160 a does not enter between the inner pipe 142 and the inner wall of the liquid transport line 150. It is preferable to integrally form the inner pipe 142 with the main body 141.

Materials of the evaporator case 120 provided with the first and the second case parts 121 and 122 preferably include a metal or an alloy so as to ensure the strength and sealing reliability. The first and the second case parts 121 and 122 are joined and fixed to each other by a method selected from among various methods capable of ensuring the sealing reliability, such as welding, brazing, and resin adhesion.

Further, the materials of the evaporator case 120 preferably include a metal or an alloy with high heat conductivity, such as oxygen-free copper, a copper alloy, aluminum, and an aluminum alloy, in order to transfer heat from the cooling-target heat generating element to the entire evaporator case 120. However, the evaporator case 120 may be made of a metal or an alloy with relatively low heat conductivity (for example, an iron based alloy such as stainless steel, or a titanium alloy) according to, for example, the size and/or required cooling capacities of the evaporator case 120.

Materials of the manifold 140 need to have lower heat conductivity than the evaporator case 120 so as to obtain heat insulation. Although the manifold 140 having lower heat conductivity is more preferable, the manifold 140 having a heat conductivity of 1 W/mK or lower achieves a significant heat insulation effect. The heat conductivity of 1 W/mK or lower is one to several orders of magnitude less compared to, for example, the case where the evaporator case 120 is made of copper (about 380 W/mK) and the case where the evaporator case 120 is made of stainless steel (about 16 W/mK). This may create a significant temperature difference between the outer wall and the inner wall of the manifold 140. As a result, the working fluid 160 a having flowed into the evaporator case 120 is effectively insulated from the evaporator case 120, which prevents the working fluid 160 a from evaporating before reaching the wicks 130.

For example, the materials of the manifold 140 may include a resin, such as a fluorine resin, a nylon resin, a PEEK (polyether ether ketone) resin, a polypropylene resin, and a polyacetal resin. As an example, MC nylon (registered trademark of Quadrant Polypenco Japan Ltd.) has a heat conductivity of about 0.2 W/mK, which is about 1/1900 that of copper and about 1/80 that of stainless steel, and therefore, MC nylon (registered trademark) with a thickness of, for example, even 1 to several mm achieves heat insulation. The manifold 140 may be a porous body made of a resin selected from the above-mentioned resins.

Although being possibly selected from various types of porous bodies, such as metal wicks, carbon wicks, and resin wicks, the wicks 130 are preferably resin wicks because they are easy to ensure close contact with the evaporator case 120 and have lower heat conductivity than other types of wicks. If the wicks have high heat conductivity, heat may be transferred to the inner periphery of the wicks at which air bubbles are generated and may have a similar effect as the generation of air bubbles before the working fluid reaches the wicks. Therefore, using resin wicks may prevent the generation of air bubbles on the inner periphery side of the wicks. Preferable materials of resin wicks include, for example, a fluorine resin, a PEEK resin, a polypropylene resin, and a polyacetal resin.

The wicks 130 and at least part of the manifold 140 may be made of the same porous resin. In that case, for example, the wicks 130 and the at least part of the manifold 140 are integrally molded and the remaining part of the manifold 140 is designed to be a simple structure, allowing easy manufacturing.

Even in the case of being applied to a small plate-type evaporator, the above-described configuration reduces or prevents generation of air bubbles due to evaporation of the working fluid before reaching the wicks, which enables stable operation of the loop heat pipe to maintain the cooling performance.

The evaporator 110 of FIGS. 3A through 3D includes two wicks 130, however, three or more wicks may be provided. According to the number of wicks, the number of outlets 144 and the internal branching structure of the manifold 140 need to be changed.

In addition, for an evaporator including a single wick, a liquid supply duct with low heat conductivity, corresponding to a manifold may be provided. FIG. 4 illustrates an evaporator 210 of a loop heat pipe including a single wick, according to another embodiment. In the following description of the evaporator 210, a detailed description of those features common to the evaporator 110 of FIGS. 3A through 3D is omitted.

The evaporator 210 includes a first case part 221 communicating with a vapor transport line 255; a second case part 222 communicating with a liquid transport line 250; a single wick 230; and a liquid supply duct 240. The first and the second case parts 221 and 222 are coupled to each other to form a single evaporator case for housing the wick 230 and the liquid supply duct 240.

A hole 223 for housing the wick 230 is provided in the first case part 221. The second case part 222 may house the liquid supply duct (manifold) 240. However, in this embodiment, no limitation is imposed on the structures of the two case parts 221 and 222 to form the evaporator case as long as the case parts 221 and 222 can be coupled to each other after the wick 230 and the liquid supply duct 240 are housed therein.

The wick 230 has an internal opening to serve as a liquid supply path 231 for supplying working fluid to the wick 230, and has multiple vapor discharge grooves (simply “grooves”) 232 on the outer periphery. The grooves 232 may run the entire length of the wick 230 along the flow direction of the working fluid.

The liquid supply duct 240 insulates the working fluid before reaching the wick 230 from the evaporator case (221 and 222), and also supplies, to the wick 230, the working fluid having flowed in from the liquid transport line 250. The liquid supply duct 240 includes a main body 241 disposed inside the evaporator case and may also include, on an as-needed basis, an inner pipe 242 extending inside the liquid transport line 250, as illustrated. The main body 241 of the liquid supply duct 240 may include an outer wall disposed along the inner wall of the evaporator case and an opening defined by the outer wall. Alternatively, the liquid supply duct 240 may include one or more piping systems for distributing the working fluid to the entire wick 230.

Materials of the first case part 221, the second case part 222, the wick 230, and the liquid supply duct 240 may be the same as those of the corresponding components (121, 122, 130, and 140, respectively) of the evaporator 110. For example, the materials of the first and the second case parts 221 and 222 include a metal or an alloy, the materials of the wick 230 include a porous resin, and the materials of the liquid supply duct 240 include a resin.

As is the case with the above-described evaporator 110, the evaporator 210 also reduces or prevents generation of air bubbles due to evaporation of the working fluid before reaching the wick 230, which enables stable operation of the loop heat pipe to maintain the cooling performance.

Note however that the configuration of the evaporator 210 that includes the single wick 230 results in a reduction in the number of components and simplification of component processing and/or assembly, which in turn reduces the manufacturing costs. On the other hand, the configuration of the evaporator 110 that includes the multiple wicks 130 and the multiple holes 123 for individually housing the corresponding wicks 130 results in an increase in the contact area between the wicks 130 and the evaporator case 120. In addition, the separation wall 124 between the multiple holes 123 acts as a heat transfer path, and therefore, heat received from the heat generating element is further uniformly transferred to the entire evaporator case 120. As a result, the evaporator 110 has an advantage over the evaporator 210 in the cooling performance of the evaporator itself and, therefore, in the cooling performance of the loop heat pipe.

Next, an evaporator 310 of a loop heat pipe according to another embodiment is described with reference to FIG. 5. FIG. 5 illustrates a cross-sectional view of the evaporator 310, as in FIG. 3C. In the following description of the evaporator 310, a detailed description of those features common to the evaporator 110 of FIGS. 3A through 3D is omitted.

The evaporator 310 includes a first case part 321 communicated with a vapor transport line 355; a second case part 322 communicated with a liquid transport line 350; at least one wick 330; and a liquid supply duct 340. The first and the second case parts 321 and 322 are coupled to each other to form a single evaporator case 320 for housing the wick 330 and the liquid supply duct 340.

The wick 330 has an internal opening to serve as a liquid supply path 331 for supplying working fluid 360 a to the wick 330, and has multiple vapor discharge grooves (simply “grooves”) 332 on the outer periphery.

The liquid supply duct 340 insulates, from the evaporator case 320, the working fluid 360 a having flowed in from the liquid transport line 350, and also supplies the working fluid 360 a to the wick 330. In the case where the evaporator 310 has multiple wicks 330, the liquid supply duct 340 takes the form of a manifold. The liquid supply duct 340 may include, on an as-needed basis, an inner pipe (not illustrated) extending inside the liquid transport line 350.

The first case part 321 and the second case part 322 may be made of different materials. Materials of the first case part 321 for housing the wick 330 therein preferably include a metal or an alloy with high heat conductivity, such as oxygen-free copper, a copper alloy, aluminum, and an aluminum alloy, in order to transfer heat from the cooling-target heat generating element to the entire evaporator case 320. Materials of the second case part 322 for housing the liquid supply duct 340 therein have lower heat conductivity than the materials of the first case part 321. In addition, the second case part 322 is preferably made of a metal or an alloy in terms of sealing reliability of the evaporator case 320. For example, the second case part 322 may be made of a metal or an alloy with relatively low heat conductivity, for example, an iron based alloy such as stainless steel, or a titanium alloy.

It is preferable that the boundary between the first case part 321 and the second case part 322 of the evaporator case 320 be substantially aligned to the boundary between the liquid supply duct 340 and the wick 330. This is in order to achieve heat transfer to the entire contact area between the wick 330 and the evaporator case 320 and obtain heat insulation for the working fluid 360 a before reaching the wicks 330.

The first and the second case parts 321 and 322 are joined to each other by a method selected from among various methods capable of ensuring the sealing reliability, such as welding, brazing, and resin adhesion.

Materials of the wick 330 and the liquid supply duct 340 may be the same as those of the corresponding components (130 and 140, respectively) of the evaporator 110. For example, the materials of the wick 330 include a porous resin, and the materials of the liquid supply duct 340 include a resin.

As is the case with the above-described evaporator 110, the evaporator 310 also reduces or prevents generation of air bubbles due to evaporation of the working fluid 360 a before reaching the wick 330, which enables stable operation of the loop heat pipe to maintain the cooling performance. Note however that because the second case part 322 is made of materials with lower heat conductivity than those of the first case part 321, the evaporator 310 enhances the effect of reducing the evaporation of the working fluid 360 a before reaching the wick 330 and, therefore, further stabilizes the operation of the loop heat pipe. Note that since the first case part 321 is made of materials with high heat conductivity, the cooling performance of the loop heat pipe is not impaired.

Next, an electronic apparatus 400 according to an embodiment is described with reference to FIGS. 6A through 6C. FIGS. 6B and 6C are cross-sectional views taken along the C-C′ line and the D-D′ line, respectively, of FIG. 6A, illustrating an example of mounting an evaporator on a heat generating element of the electronic apparatus 400. Note that the D-D′ cross section of FIG. 6C is selected as a cross section passing substantially through the center of one of wicks and including neither a liquid transport line nor a vapor transport line.

The electronic apparatus 400 includes an electronic component (hereinafter, sometimes referred to as the “heat generating element”) 470 which is a heat generating element; and a loop heat pipe 405 for cooling the electronic component 470.

The loop heat pipe 405 includes an evaporator 410 which is, for example, any one of the above-described evaporators 110, 210, and 310; and a condenser 461 for dissipating heat and condensing vaporized working fluid generated by the evaporator 410 into liquid working fluid. The condenser 461 is cooled, for example, by sending air 462 from an air blower to heat dissipating fins of the condenser 461 or by placing the condenser 461 into a liquid cooled to ambient temperature or lower. The vaporized working fluid is supplied to the condenser 461 from the evaporator 410 via a vapor transport line 455. The working fluid from the condenser 461 is supplied to the evaporator 410 via a liquid transport line 450. The loop heat pipe 405 typically has a reservoir tank 463 in the middle of the liquid transport line 450, disposed before the evaporator 410. The reservoir tank 463 stores therein working fluid needed at startup. The working fluid may be, for example, water, ethanol, R141B, n-Pentane, acetone, butane, or ammonia.

The heat generating element 470 of the electronic apparatus 400 is, for example, a semiconductor device (such as a CPU), and is mounted on a wiring substrate 475 (such as a mother board) of the electronic apparatus 400. The evaporator 410 may be mounted and fixed to the heat generating element 470 by, for example, screwing a pressing fastener (not illustrated) onto the wiring substrate 475. A high heat conductive material 480, such as a thermal grease, may be disposed between the heat generating element 470 and the evaporator 410. Note that the single evaporator 410 may be used to cool multiple heat generating elements.

As illustrated in FIG. 6C, the heat generating element 470 may be disposed offset to the vapor transport line side (right-hand side in FIG. 6C) relative to the evaporator 410. That is, the evaporator 410 may be mounted on the heat generating element 470 in such a manner that the center of the heat generating element 470 is located on the vapor transport line side away from the center of the evaporator case 420. Such an offset results in an increase in the distance between the heat generating element 470 and working fluid 460 a before reaching the wicks, which may in turn prevent the working fluid 460 a from evaporating before reaching the wicks. For example, if dimensional constraints are not so much an issue, the evaporator 410 is disposed in such a manner that the manifold 440 and the heat generating element 470 do not overlap one another.

Next described are several application examples used to cool a CPU (heat generating element) with a package size of about 30 mm×30 mm.

An evaporator case used is made up of two split parts, a first case part disposed on the vapor side and a second case part disposed on the liquid side. The first case part is made of oxygen-free copper, and the second case part is made of oxygen-free copper or stainless SUS304. The outer size of the evaporator case with the first and the second case parts coupled together is about 40 mm×40 mm in planar size and about 8 mm in thickness. The small dimensions allow the evaporator case to be mounted on a CPU in a computer system with high-density packaging, such as a server and a personal computer. Two oval holes are provided parallel to each other on the inner side of the first case part. Each of the holes is about 18 mm in width (major axis) and about 6 mm in height (minor axis). A porous resin body (a resin wick) is inserted into each of the two holes.

Each of the wicks used is an about 30 mm long porous body made of PTFE (polytetrafluoroethylene). The average pore diameter of the resin wicks is about 2 μm and the porosity is about 40%. Both the thickness and width of the wicks are set to be about 100 to 200 μm larger than the dimensions of the corresponding holes on the first case part. Because the PTFE porous body is elastic, the wick having an outer size slightly larger than the wick insertion hole allows the inner wall of the first case part and the outer periphery of the wick to be in close contact with each other. On the inner periphery of each of the resin wicks, an oval hole about 2 mm in height and about 14 mm in width is provided to serve as a liquid supply path for receiving working fluid supplied from a liquid transport line via a manifold. In addition, multiple grooves, each 1 mm in depth and 1 mm in width, are formed on the outer periphery of the wicks. Vaporized working fluid is released from the surface of the grooves and flows through the grooves, and is then discharged to a vapor transport line.

A resin manifold made of MC nylon (registered trademark) is disposed in the evaporator case with no space between the manifold and the resin wicks. The manifold distributes the working fluid having flowed in from the liquid transport line to the two resin wicks without letting the working fluid run out of the manifold. That is, the working fluid having flowed into the evaporator is guided into the resin wicks via the resin manifold, without coming in contact with the metal evaporator case. As a result, heat transfer from the metal evaporator case to the working fluid is reduced, which may prevent generation of air bubbles. The resin manifold has a wall thickness of about 1 mm. Since the heat conductivity of MC nylon (registered trademark) is 0.2 W/mK, which is a fraction of several tens to several thousands of that of copper (380 W/mK) and that of SUS304 (16 W/mK), such a thin MC nylon material is still effective as a heat insulator.

For some of the application examples, the heat insulation resin of the manifold is extended to the liquid transport line side to thereby form an inner pipe, which is inserted into the liquid transport line.

To assemble the evaporator, the resin wicks and the resin manifold are inserted into the first and the second case parts. Subsequently, the first and the second case parts are sealed together to thereby complete the assembly. For the application examples, the first and the second case parts are sealed by laser welding.

After the evaporator is assembled in the above-described manner, the evaporator, the vapor transport line, a condenser unit provided with heat dissipating fins, and the liquid transport line are connected one to the other in a loop by welding, and then working fluid is enclosed therein. As an example, copper pipes having an outer diameter (φ) about 4 mm and an inner diameter (φ) about 3 mm may be used as the vapor transport line and the liquid transport line. The entire length of the copper pipes may be, for example, about 900 mm. The working fluid used is n-Pentane. The condenser is cooled by sending air from an air blower to the heat dissipating fins of the condenser unit.

Subsequently, the evaporator is thermally bonded onto the CPU via a thermal grease (for example, W4500 produced by COSMO OIL Co., Ltd.). The evaporator is fixed onto the CPU by screwing a pressing fastener. At this point, in order to provide a longer distance between the working fluid having flowed into the evaporator and the CPU, the evaporator is disposed in such a manner that the center of the CPU is located on the vapor transport line side away from the center of the evaporator case.

An experiment was carried out to examine the operation of a loop heat pipe configured in the above-described manner. FIG. 7 illustrates application examples (a) to (c) used for the operation examination.

An evaporator 510 of application example (a) has a structure in which PTFE wicks 530 and an MC nylon manifold 540 are disposed inside a metal case 520 including a first case part 521 and a second case part 522 both made of oxygen-free copper. An evaporator 510′ of application example (b) includes a manifold 540′ formed by integrally molding the MC nylon manifold 540 of application example (a) and an MC nylon inner pipe 542. The 20 mm long MC nylon pipe 542 (outer diameter (φ) 4 mm and inner diameter (φ) 3 mm) is inserted into the end portion of the liquid transport line (outer diameter (φ) 5 mm and inner diameter (φ) 4 mm). An evaporator 510″ of application example (c) includes an evaporator case 520″ which is identical to the evaporator case 520 of application example (a) except for a second case part 522″ made of stainless SUS304 in place of oxygen-free copper. The SUS304 second case part 522″ corresponds to about 8 mm of the 40-mm-long evaporator case 520″.

As for the application examples (a) through (c), the offset between the CPU 570 and the corresponding evaporators 510, 510′, and 510″ is about 4 mm. The offset is provided in order to prevent, in application example (c), the 30-mm-long CPU 570 and the 8-mm-long second case part 522″ from overlapping each other.

By way of comparison, comparison examples (d) and (e) having no resin manifold (both not illustrated) were prepared. Comparison examples (d) and (e) have structures identical to those of application examples (a) and (c), respectively, except for simply not having the resin manifold 540.

For these examples (a) through (e), confirmation of operation capabilities of the loop heat pipe as well as measurement of heat transfer resistance of the loop heat pipe were carried out under fixed conditions, using the amount of heat generated by the CPU as a parameter (FIG. 8). The heat transfer resistance is calculated as follows. The average temperature of the condenser (an average value of the inlet and outlet temperatures) is subtracted from the temperature of the heat receiving surface of the evaporator to obtain a temperature difference. Then, the temperature difference is divided by the amount of heat generated by the CPU to obtain the heat transfer resistance.

As for comparison examples (d) and (e) having no resin manifold, the working fluid came to a boil and evaporated around a part of the evaporator at which the liquid transport line is connected to the evaporator case and the working fluid flows in. As a result, the circulation of the working fluid was unstable, and the loop heat pipe did not operate properly.

On the other hand, as for application examples (a) through (c) having a resin manifold, the circulation of the working fluid was stable, and the loop heat pipe operated properly. FIG. 8 illustrates evaluation results in terms of the heat transfer resistance of application examples (a) through (c). Based on the results illustrated in FIG. 8 and the fact that comparison examples (d) and (e) did not operate properly, it is understood that the manifold with low heat conductivity contributes largely to stabilizing operation of the loop heat pipe. It has been also found that the combination of the manifold and the inner pipe (application example (b)) and the combination of the manifold and the second case part with relatively low heat conductivity (application example (c)) may further improve the cooling performance of the loop heat pipe. These results indicate that it is possible to make the evaporator of the loop heat pipe even smaller and thinner, which expands the possibility of cooling design for high-heat-generating electronic components mounted on electronic apparatuses, such as computer systems with high-density packaging.

As in application examples (a) through (c) described above, a structure with the evaporator case made of a metal has good pressure resistance and prevents leakage of the working fluid in the long term and, therefore, largely contributes to offering a reliable cooling system.

While the embodiments have been described in detail, it should be understood that the present invention is not limited to these specific embodiments, and various changes and modification may be made to the particular examples without departing from the scope of the claims appended hereto. For example, although the embodiments above are described based on the plate-type evaporator, also in other types of evaporators such as a cylindrical evaporator, the working fluid may be supplied to a single or multiple wicks via a liquid supply duct with low heat conductivity, on an as-needed basis.

The above-described embodiments have the following advantageous effects. Heat transfer from the evaporator case to the working fluid having flowed into the evaporator is reduced, which prevents the working fluid from evaporating before reaching the wicks. As a result, the capillary force of the wicks is maintained, allowing stable circulation of the working fluid. This in turn achieves efficient cooling of the electronic component in the electronic apparatus.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the invention. Although the embodiments of the present disclosure have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A loop heat pipe comprising: a liquid transport line; an evaporator; a vapor transport line; and a condenser, the liquid transport line, the evaporator, the vapor transport line, and the condenser being connected to one another to circulate a working fluid, the evaporator including a case including a liquid flow inlet and a vapor flow outlet, at least one porous body disposed inside the case and configured to guide the working fluid in a liquid phase inward of the case, and a liquid supply duct disposed inside the case and configured to guide the working fluid in the liquid phase into the at least one porous body from the liquid flow inlet, wherein the liquid supply duct is made of a material having lower heat conductivity than a material of the case.
 2. The loop heat pipe as claimed in claim 1, wherein the material of the case is a metal or an alloy, and the material of the liquid supply duct has a heat conductivity of 1 W/mK or lower.
 3. The loop heat pipe as claimed in claim 1, wherein the material of the case is a metal or an alloy, and the material of the liquid supply duct is a resin.
 4. The loop heat pipe as claimed in claim 3, wherein the material of the liquid supply duct is selected from a group consisting of a fluorine resin, a nylon resin, a polyether ether ketone resin, a polypropylene resin, and a polyacetal resin.
 5. The loop heat pipe as claimed in claim 1, wherein the at least one porous body is made of a porous resin.
 6. The loop heat pipe as claimed in claim 5, wherein the porous resin is selected from a group consisting of a fluorine resin, a polyether ether ketone resin, a polypropylene resin, and a polyacetal resin.
 7. The loop heat pipe as claimed in claim 5, wherein the at least one porous body and the liquid supply duct are made of a same porous resin.
 8. The loop heat pipe as claimed in claim 1, wherein the liquid supply duct extends continuously from the liquid flow inlet to the at least one porous body.
 9. The loop heat pipe as claimed in claim 1, wherein the liquid supply duct includes a tubular part extending inside the liquid transport line.
 10. The loop heat pipe as claimed in claim 9, wherein the tubular part is disposed in close contact with an inner wall of the liquid transport line.
 11. The loop heat pipe as claimed in claim 1, wherein the at least one porous body is provided as plural of the porous bodies, and the liquid supply duct is a manifold for distributing the working fluid in the liquid phase to the plural porous bodies.
 12. The loop heat pipe as claimed in claim 1, wherein the at least one porous body includes, on an outer periphery, a vapor discharge groove running an entire length of the at least one porous body from a side closer to the liquid transport line to a side closer to the vapor transport line, and an end portion of the vapor discharge groove on the side closer to the liquid transport line is terminated by a wall surface of the liquid supply duct.
 13. The loop heat pipe as claimed in claim 1, wherein the case has a plate-like outer shape.
 14. The loop heat pipe as claimed in claim 1, wherein the case includes a first part disposed in contact with the at least one porous body and a second part disposed on a side closer to the liquid supply duct and housing at least part of the liquid supply duct, and the second part is made of a material having lower heat conductivity than a material of the first part.
 15. The loop heat pipe as claimed in claim 14, wherein the material of the first part is selected from a group consisting of oxygen-free copper, a copper alloy, aluminum, and an aluminum alloy.
 16. The loop heat pipe as claimed in claim 14, wherein the material of the second part is selected from a group consisting of an iron based alloy and a titanium alloy.
 17. An electronic apparatus comprising: the loop heat pipe as claimed in claim 1; and an electronic component thermally bonded to the evaporator of the loop heat pipe.
 18. The electronic apparatus as claimed in claim 17, wherein on a bonded plane between the electronic component and the case of the evaporator, a center of the electronic component is offset, in relation to a center of the case, in a direction opposite from a direction to the liquid flow inlet.
 19. The electronic apparatus as claimed in claim 18, wherein the case of the evaporator includes a first part disposed in contact with the at least one porous body and a second part disposed on a side closer to the liquid supply duct and made of a material having lower heat conductivity than a material of the first part, and the electronic component is bonded to the first part. 